1. Field of the Invention
The present invention relates to a solid-state laser apparatus, and more specifically to a mode-locked solid-state laser apparatus which is easily downsized and capable of short-pulse operation.
2. Description of the Related Art
Solid-state lasers in which a semiconductor laser (LD) is used as the excitation light source and a solid-state laser medium (laser crystal, ceramics, glass, or the like) doped with rare earth ions or transition metal ions is excited by the light source have been actively developed. Among them, short pulse lasers that generate so-called short pulse beams in the range from picoseconds to femtoseconds have been sought and proposed in many application fields including medicine, biology, machine industry, and measurement, and some of them are put into practical use after demonstration.
This type of lasers generates short pulses by so-called mode locking. To put it briefly, the mode locking is a phenomenon in laser oscillation where all phases of multi-longitudinal modes are locked (relative phase difference=0) in the frequency domain, and the pulses become extremely short in the time domain due to multimode interference between longitudinal modes. In the field of solid-state lasers, mode locking by semiconductor saturable absorbing mirror (SESAM) has been actively developed since it is simple, low cost, small, and self initiating.
Particularly, in soliton mode locking which is one of the mode locking regimes, the combination of negative group velocity dispersion in the laser resonator and self phase modulation mainly in the laser medium enables generation of pulses in the femtosecond region. More specifically, the soliton mode locking is a regime in which mode locking is initiated and the pulses are maintained/stabilized by the semiconductor saturable absorbing mirror, at the same time the mode locked pulses become sharp through soliton pulse forming which occurs by balancing the negative group velocity dispersion with self phase modulation, thereby stable pulse generation becomes possible (Optics Letters, Vol. 25, No. 15, pp. 1119-1121, 2000 (Non-Patent Document 1), and Journal of Optical Society of America, Vol. 16, No. 1, pp. 46-56, 1999 (Non-Patent Document 3); definition of soliton mode locking).
Basically, a solid-state laser apparatus that realizes the soliton mode locking includes a solid-state laser medium, a saturable absorbing mirror, and a negative group velocity dispersion element within the resonator. In the following, the negative group velocity dispersion is also simply referred to as negative dispersion. A typical configuration of conventional soliton mode-locked solid-state laser doped with Yb (solid-state laser medium is Yb:KGd(WO4)2) as described in Non-Patent Document 1 is shown in FIG. 18. In the drawing, the reference symbol 80 is an excitation light source that emits, for example, 980 nm excitation light, 81 is an input optical system provided for each of a pair of the excitation light sources 80, 83 is a solid-state laser medium, M1 and M2 are a pair of concave mirrors forming a resonator with a curvature radius of, for example, 20 cm, 84 is a concave mirror with a curvature radius of 20 cm, 85 is a SESAM, 86 and 87 are prisms forming a prism pair made of, for example, SF10 glass, 88 is a knife edge plate, and 89 is an output coupler with a transmittance of, for example, 4.3%.
As illustrated in FIG. 18, the conventional apparatus employs a configuration in which the beam radius of the laser oscillation light is focused separately at the solid-state medium and SESAM by the concave mirrors M1, M2, and 84, in order to reduce the beam radius ωL at the laser medium and the beam radius ωA at the SESAM.
The spot sizes on the laser medium and SESAM are reduced for the following two reasons. First reason is to reduce the threshold value of laser oscillation, and the second reason is to satisfy the soliton mode locking condition.
The first reason will now be described. The laser oscillation threshold Pth is represented by Formula (1) below as described, for example, in Applied Optics, Vol. 36, No. 9, pp. 1867-1874, 1997 (Non-Patent Document 2).
                              P          th                =                                            π              ⁢                                                          ⁢              h              ⁢                                                          ⁢                                                v                  P                                ⁡                                  (                                                            ω                      L                      2                                        +                                          ω                      P                      2                                                        )                                                                    4              ⁢              σ              ⁢                                                          ⁢                                                τη                  a                                ⁡                                  (                                                            f                      1                                        +                                          f                      2                                                        )                                                              ⁢                      (                                          L                i                            +                              T                                  0                  ⁢                  C                                            +                              2                ⁢                                  f                  1                                ⁢                σ                ⁢                                                                  ⁢                                  N                  0                                ⁢                l                ⁢                                                                  ⁢                s                                      )                                              (        1        )            
where, ωp is the excitation light beam radius at the solid-state medium, hνp is the excitation light photon energy, σ is the cross-section of stimulated emission of the solid-sate laser medium, τ is the upper level life time, ηa is the absorption efficiency, f1 is the lower level occupancy, f2 is the upper level occupancy, Li is the intracavity loss of the resonator, Toc is the output mirror transmittance, N0 is the rare earth ion dope density, and ls is the crystal length.
Formula (1) above shows that it is only necessary to reduce the oscillation beam radius ωL and excitation light beam radius ωP in the solid-state laser medium in order to reduce the oscillation threshold.
Next, the second reason, that is, the soliton mode locking condition will be described. As described in Non-Patent Document 3, it is pointed out that mode locking mixed with Q-switching operation (Q-switched mode locking) occurs in a soliton mode-locked laser under a certain condition. The Q-switched mode locking is an operation mode in which a mode-locked pulse train (frequency in the range from 10 MHz to 1 GHz, pulse width in the range from picoseconds to femtoseconds) is disposed in a long pulse of Q-switched pulse (frequency in the range from 1 KHz to several 100 KHz, pulse width in the range from microseconds to nanoseconds).
This operation mode is generally undesirable other than for energy application due to instability in output, pulse width, and pulse period. According to the Non-Patent Document 3, the condition not to cause Q switching in the soliton mode locking using a saturable absorption mirror are expressed by Formula (2) below.Fsat,L·AeffL,·g·K2EP3+EP2>Fsat,L·AeffL,·Fsat,A·Aeff,A·ΔR   (2)
where: Ep is the intracavity pulse energy; ΔR is the modulation depth of the saturable absorption mirror; Fsat,A is the saturated fluence of the saturable absorption mirror; Fsat,L (=hν/σ) is the saturated fluence of the laser medium; hν is the oscillation laser photon energy; Aeff,A (=πωA2) is the oscillation light beam cross section at the saturable absorption mirror; Aeff,L (=πωL2) is the oscillation light beam cross section at the laser medium, g is the laser gain of the laser medium, and K is the following.
  K  =                    4        ⁢        π        ⁢                                  ⁢                  n          2                ⁢        l        ⁢                                  ⁢        s                                        D                          ⁢                  A                      eff            ,            L                          ⁢                  λ          0                ⁢        Δ        ⁢                                  ⁢                  v          G                      ⁢          0.315      1.76      
(where, n2 is the nonlinear refraction index of the laser medium, D is the total group velocity dispersion of the entire resonator for one round trip (D<0), λ0 is the center frequency of the oscillation light, and ΔνG is the gain bandwidth.) Note that, in Formula (2) above, the solution of Ep when the left term corresponds to the right term is the mode locking threshold, and to satisfy Formula (2) means to set the Ep to a value greater than the mode locking threshold.
Formula (2) shows that it is necessary to reduce the beam cross section at the laser medium and the beam cross-section at the SESAM, and/or to increase the intracavity pulse energy Ep in order to avoid generation of the Q-switched mode locking.
From the two conditions with respect to the laser oscillation threshold and CW mode locking threshold described above, it is necessary to reduce the beam cross section on the laser medium and SESAM. Most of the conventional mode-locked solid-state lasers employ a configuration in which a laser medium is placed between two concave mirrors (M1 and M2 in the example of FIG. 18, generally those with a curvature radius of about 100 to 200 mm are used) to narrow down the beam, and at the same time beam focusing is performed on the SESAM by a concave mirror.
The conventional mode-locked solid-state laser apparatuses structured in the manner as described above require at least three concave mirrors, and in some cases further require a plurality of mirrors for beam replication. This increases the number of optical components of the mode-locked solid-state laser, and the apparatus cost is increased.
The distance between the concave mirror and the laser medium, and between the concave mirror and SESAM is usually set to a value about half the curvature radius, which alone amounts to about 150 mm (when curvature radius is 100 mm) to 300 mm (when curvature radius is 200 mm) in total. Further, when considering an insertion space for a negative dispersion element or the like, the resonator requires a length of about 500 mm to 1 m, so that the laser apparatus inevitably become large. In the configuration shown in FIG. 18, the negative dispersion is provided by the prism pair of prisms 86 and 87, and the distance between the prisms is 450 mm. Generally, when a resonator of a meter length is formed in a solid-state laser, stable operation is difficult. For this reason, the conventional apparatuses have low laser oscillation stability.
In the mean time, U.S. Pat. No. 7,106,764 (Patent Document 1) proposes a mode-locked solid-state laser apparatus downsized by disposing the solid-state laser medium and SESAM in close proximity to each other. Further, Japanese Unexamined Patent Publication No. 11(1999)-168252 (Patent Document 2) proposes a mode-locked solid-state laser apparatus in which a saturable absorption mirror is formed on a solid-state laser medium by coating and a negative dispersion mirror doubles as the output mirror to reduce the number of optical components and to downsize the apparatus.
Disposition of the saturable absorption mirror, which is a reflection mirror, and the laser medium in close proximity or in contact with each other as in Patent Document 1 and Patent Document 2, however, causes the following problems.
It is known from the following literatures that so-called spatial hole burning effects occur differently in the resonator depending on the position of the optical axis of the laser medium acting as gain medium, which are then combined with the mode locking phenomenon and influence the stability of the mode locking: Applied Physics B, Vol. 72, pp. 267-278, 2001 (Reference Document 1), Applied Physics B, Vol. 61, pp. 429-437, 1995, and Applied Physics B, Vol. 61, pp. 569-579, 1995.
On the surface of a reflection mirror constituting the resonator, phase jumping occurs in the lightwave field inside thereof which produces a “node” where the field intensity falls to 0. Where the laser medium is disposed adjacent to the reflection mirror, a spatial stripe pattern of laser lightwave intensity is developed in the laser medium by the phase jumping, which is called the spatial hole burning.
It is known that there is the following difference between a case in which the laser medium is disposed adjacent to the reflection mirror and a case in which it is disposed in the middle of the resonator. The literature, Optics Letters, Vol. 25, No. 11, pp. 859-861, 2000, describes that stable mode locking is obtained only with a pulse width adjacent to 700 fsec in a LD-excited thin disk Yb:YAG (Yb:Y3Al5O12) laser in which the reflection mirror and solid-state laser medium is disposed closely, and stable CW mode locking is not obtained with the other pulse widths. On the other hand, it is known that a wider pulse width range (90 psec to 800 fsec) can be realized in a system with reduced spatial hole burning (physically equivalent to the case in which the laser medium is spaced apart from the reflection mirror) as described, for example, in the literature, Optics Letters, Vol. 26, No. 6, pp. 379-381, 2001.
Reference Document 1 describes that where the laser medium is disposed adjacent to the reflection mirror, a depression occurs in the gain spectrum, and it causes instability of the soliton pulses moving round inside the resonator. More specifically, the spatial hole burning effects appear more strongly adjacent to the reflection mirror, so that the gain stripe pattern developed in the laser medium by the spatial hole burning leads to modulation of the gain spectrum in the frequency domain. Eventually, the gain is preferentially given to phenomena (shift pulse, double pulse, CW background) competing with the desired pulse. This causes the desired soliton pulse to lose the competition and the pulse phenomena described above to occur, leading to the instability.
Accordingly, where the reflection mirror of saturable absorption mirror and laser medium are disposed in close proximity or in contact with each other as in Patent Document 1 and Patent Document 2, it is thought that the spatial hole burning occurs significantly, leading to significant instability of the soliton pulse.
Patent Document 1 and Patent Document 2, however, do not describe the influence of spatial hole burning to the mode stability in any way, and do not disclose any measure for stabilizing the mode. In the mean time, Reference Document 1 discusses only the 700 fsec pulse operation in 1030 nm oscillation in a LD-excited Yb:YAG laser. Furthermore, it discusses only the operation in a high output power region of several tens of watts. Mode stabilization conditions for other transition elements that can be expected to provide short pulses in the range from 100 to 200 fsec, for example, 1050 nm oscillation of Yb:YAG, Yb:KYW(Yb:KY(WO4)2) crystal, Yb:KGW(Yb:KGd(WO4)2) crystal, Yb:Y2O3, Yb:Sc2O3, Yb:Lu2O3, Er,Yb:glass, Nd:glass are not discussed in any way.
That is, although configurations for downsizing mode-locked solid-state laser apparatuses are proposed, these documents do not clearly describe the conditions for stably generating soliton pulses in the downsized configurations. Consequently, it has been difficult to realize a small mode-locked solid-state laser apparatus capable of providing stable mode-locked oscillation.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to define the conditions of stabilizing mode-locked operation and to provide a small, low cost, and highly stable solid-state laser apparatus capable of realizing femtosecond CW mode locking.